DEEP-SEA RESEARCH I 43, 1567-1580

Growth rates and production of heterotrophic bacteria and phytoplankton in the North Pacific subtropical gyre

David R. Jones¹, David M. Karl² and Edward A. Laws²

¹Grice Marine Biological Laboratory, University of Charleston, 205 Fort Johnson, Charleston, SC 29412, U.S.A.

²University of Hawaii, School of Ocean and Earth Science and Technology, Department of Oceanography, 1000 Pope Road, Honolulu, Hawaii 96822, U.S.A.

In field work conducted at 26N, 155W, in the North Pacific subtropical gyre, phytoplankton growth rates u_p estimated from ¹⁴C labeling of chlorophyll a (chl a) averaged approximately one doubling per day in the euphotic zone (0-150 in). Microbial (microalgal plus heterotrophic bacterial) growth rates u_m calculated from the incorporation of ³H-adenine into DNA were comparable to or exceeded phytoplankton growth rates at most depths in the euphotic zone. Photosynthetic rates averaged 727 mg C m^-2 day^-1. Phytoplankton carbon biomass, calculated from ¹⁴C labeling of chl a, averaged 7.2 mg m^-3 in the euphotic zone. Vertical profiles of particulate DNA and ATP suggested that no more than 15% of particulate DNA was associated with actively growing cells. Heterotrophic bacterial carbon biomass was estimated from a two-year average at station ALOHA (22 45'N, 158'W) of flow cytometric counts of unpigmented. bacteria-size particles which bound DAPI on the assumption that 15% of the particles were actively growing cells and that heterotrophic bacterial cells contained 20 fg C cell^-1. The heterotrophic bacterial carbon so calculated averaged 1.1 mg m^-3 in the euphotic zone. Heterotrophic bacterial production was estimated to be 164 mg C m^-2 day^-1, or 23% of the calculated photosynthetic rate. Estimated heterotrophic bacterial growth rates averaged 0.97 day^-1 in the euphoric zone and reached 4.7 day^-1 at a depth of 20 m. Most heterotrophic bacterial production occurred in the upper 40 m of the euphotic zone, suggesting that direct excretion by phytoplankton, perhaps due to photorespiration or ultraviolet fight effects, was a significant source of dissolved organic carbon for the bacteria.